This invention relates to electrophotography and more particularly, to an improved overcoated electrophotographic imaging member and method of using the electrophotographic imaging member.
Generally, electrophotographic imaging processes involve the formation and development of electrostatic latent images on the imaging surface of a photoconductive member. The photoconductive member is usually imaged by uniformly electrostatically charging the imaging surface in the dark and exposing the member to a pattern of activating electromagnetic radiation such as light, to selectively dissipate the charge in the illuminated areas of the member to form an electrostatic latent image on the imaging surface. The electrostatic latent image is then developed with a developer composition containing toner particles which are attracted to the photoconductive member in image configuration. The resulting toner image is often transferred to a suitable receiving member such as paper.
The photoconductive members include single or multiple layered devices comprising homogeneous or heterogeneous inorganic or organic compositions and the like. One example of a photoconductive member containing a heterogeneous composition is described in U.S. Pat. No. 3,121,006 wherein finely divided particles of a photoconductive inorganic compound is dispersed in an electrically insulating organic resin binder. The commercial embodiment usually comprises a paper backing containing a coating thereon of a binder layer comprising particles of zinc oxide uniformly dispersed therein. Useful binder materials disclosed therein include those which are incapable of transporting for any significant distance injected charge carriers generated by the photoconductive particles. Thus, the photoconductive particles must be in substantially contiguous particle to particle contact throughout the layer for the purpose of permitting charge dissipation required for cyclic operation. Thus, about 50 percent by volume of photoconductive particles is usually necessary in order to obtain sufficient photoconductive particle to particle contact for rapid discharge. These relatively high photoconductive concentrations can adversely affect the physical continuity of resin binder and can significantly reduce the mechanical strength of the binder layer.
Other known photoconductive compositions include amorphous selenium, halogen doped amorphous selenium, amorphs including selenium arsenic, selenium tellurium, selenium arsenic antimony doped selenium alloys, cadmium sulfide and the like. Generally, these inorganic photoconductive materials are deposited as a relatively homogeneous layer on suitable conductive substrates. Some of these inorganic layers tend to crystallize when exposed to certain vapors that may occasionally be found in the ambient atmosphere. Moreover, the surfaces of selenium type photoreceptors are highly susceptible to scratches which print out in final copies.
Still other electrophotographic imaging members known in the art comprise a conductive substrate having deposited thereon an organic photoconductor such as a polyvinylcarbazole-2,4,7-trinitrofluorenone combination, phthalocyanines, quinacridones, pyrazolones and the like. Some of these photoreceptors, such as those containing 2,4,7-trinitrofluorenone, present health or safety issues
Recently, there has been disclosed layered photoresponsive devices comprising photogenerating layers and transport layers deposited on conductive substrates as described, for example, in U.S. Pat. No. 4,265,990 and overcoated photoresponsive materials containing a hole injecting layer, a hole transport layer, a photogenerating layer and a top coating of an insulating organic resin, as described, for example, in U.S. Pat. No. 4,251,612. Examples of photogenerating layers disclosed in these patents include trigonal selenium and various phthalocyanines and hole transport layers containing certain diamines dispersed in inactive polycarbonate resin materials. The disclosures of each of these patents, namely, U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,251,612 are incorporated herein by reference in their entirety. Other representative patents containing layered photoresponsive devices include U.S. Pat. No. 3,041,116; U.S. Pat. No. 4,115,116; U.S. Pat. No. 4,047,949 and U.S. Pat. No. 4,081,274. These patents relate to systems that require negative charging for hole transporting layers when the photogenerating layer is beneath the transport layer. Photogenerating layers overlying hole transport layers require positive charging but must be equal to or less than about 1 to 2 micrometers for adequate sensitivity and therefore wear away quite rapidly.
While the above described electrophotographic imaging members may be suitable for their intended purposes, there continues to be a need for improved devices. For example, the imaging surface of many photoconductive members is sensitive to wear, ambient fumes, scratches and deposits which adversely affect the electrophotographic properties of the imaging member.
Also, in multilayered photoreceptors comprising a charge generating layer and a charge transport layer, wear of the transport layer during image cycling limits the life of small diameter organic photoreceptor drums employed in copiers, duplicators, printers, facsimile machines and the like. With the advent of Bias Charging Rolls (BCR),and Bias Transfer Rolls (BTR) the drum wear is catastrophic. Even with the gentlest of the Bias Charging Rolls, the wear is as much as 8 to 10 micrometers in 100 kilocycles of revolutions. With the small diameter drum and duty cycle considerations 100 kilocycles of revolution translates to as little as 10,000 to 20,000 prints. The machines employing these small diameter drums do not employ exposure control. Wear results in considerable reduction of sensitivity of the device. A drum life of 50,000 or more prints (one or million drum revolution cycles) is sorely needed.
Overcoating layers have been proposed to overcome the undesirable characteristics of uncoated photoreceptors. However, many of the overcoating layers adversely affect electrophotographic performance of an electrophotographic imaging member. One type of overcoating material that has been described in the prior art is electrically insulating. For example, an insulating overcoating containing an organic high polymer and Lewis acid is described in U.S. Pat. No. 4,225,648. This overcoating may also contain other additives such as pigment, dye and hardener. An insulating overcoating containing the combination of a resin and an organic aluminum compound is described in U.S. Pat. No. 3,966,471. Apparently, the organic aluminum compound reacts with the resin to promote transfer of toner images to the receiving member. In U.S. Pat. No. 4,191,568, an insulating overcoating containing a resin and certain electron donor compounds with or without electronic acceptor compounds are mentioned. When an electrically insulating overcoating layer is employed, the thickness must be quite thin to permit discharge of the photoreceptor during exposure to activating radiation and image configuration. Further, there is a tendency for a residual charge to remain on the surface of the insulating overcoating layer after exposure. The residual voltage level intensifies as the thickness of the insulating coating is increased. This causes undesirably high background deposits in the final toner image. Moreover, scratches on imaging surfaces tend to be printed out due to the electrical differences between the scratched areas and the unscratched areas. Attempts have been made to minimize these problems by making the insulating coating as thin as possible. However, thin coatings are difficult to uniformly deposit and are subject to rapid wear. As the overcoating wears and changes in thickness, the imaging characteristics of the photoreceptor also change since charge density is dependent upon thickness.
The electrophotographic industry is feverishly searching for a tough overcoat. One durable overcoat is a cross linked polyamide (e.g. Luckamide, available from Dai Nippon Ink) containing dihydroxy biphenyl diamine (DHTBD) and dihydroxy triphenyl methane (DHTPM), and employing oxalic acid for cross linking. Although, this composition exhibits excellent electrical and wear properties, the low charge carrier mobility of this overcoat limits the overcoat thickness to less than 3 micrometers. Overcoats of this material having a thickness greater than 3 micrometers results in a severe increase on the "tails" of Photo-Induced Discharge Curve (PIDC). This severe increase on the "tails" results in loss of contrast potentials. Contrast potential is the difference in potential of photoconductor regions exposed to dark regions of the print and those exposed to the white background regions of the print. Loss of contrast potential can result in lighter images or increase in density of the white background regions of the print. Moreover, the formulation of an overcoat composition that exhibits a lower wear rate is a formidable task because the overcoat must also transport holes (without trapping), be insensitive to moisture, and not redissolve the transport layer when the overcoating is applied.
Attempts have also been made to overcome the deficiencies of overcoating layers by employing overcoating material which is less insulating to prevent electric charge from accumulating on or in the overcoating layer. Conductive overcoatings have been disclosed containing aromatic diamines. For example, the aromatic diamine is combined with an organic halogen capable of producing a free halogen in U.S. Pat. No. 4,293,630. Examples of prior art additives used to render protective overcoatings conductive including carbon black, metal powder, tetra-ammonium salt and the like are mentioned in the introduction of U.S. Pat. No. 4,191,568. Conductive overcoatings containing a resin and metal oxide particles are revealed in U.S. Pat. No. 4,426,435. The protective layer may also be made less insulating by incorporating appropriate materials such as quaternary ammonium salts or the like in the overcoating layer. However, the conductivity of such materials varies greatly due to the absorption of ambient moisture. Moreover, under very dry conditions, the conductivity of this type of overcoating layer is reduced to the extent that charge will accumulate on the outer surface of the overcoating layer with the attendant adverse effects described above with respect to insulating layers. Under humid conditions, the charge migration tends to occur laterally resulting in blurred images.
An overcoating containing a charge transport layer formed from linoleic acid and ethylene diamine is taught in U.S. Pat. No. 3,713,820. Electron acceptor compounds may be added to form a charge transfer complex thereby increasing the coating conductivity. An overcoating containing a resin and a metallocene is taught in U.S. Pat. No. 4,315,980. It appears that at least some of the resins form a charge transfer complex with ferrocene. Moreover, an electron acceptor may also be added to the overcoating layer. Further, a thin intermediate layer may be provided below the protective layer to improve electrical characteristics. The overcoatings of U.S. Pat. No. 3,713,820 and U.S. Pat. No. 4,315,980 exhibit a change in electrical conductivity by reacting with corona generated oxidizing compounds formed during charging.
In still another overcoated photoreceptor described in U.S. Pat. No. 4,515,882, the overcoat comprises an insulating film forming continuous phase comprising charge transport molecules and finely divided charge injection enabling particles dispersed in the continuous phase. Since the charge carriers giving rise to conductivity in these overcoatings emanate from the injecting particles only, the concentration of the injection particles must be higher than if the homogeneous medium surrounding the particles is also made conducting.
While some of the above-described imaging members exhibit certain desirable properties such as protecting the surface of an underlying photoconductive layer, there continues to be a need for improved overcoating layers for protecting electrophotographic imaging members.